This invention relates to the solidification of radioactive liquid waste in a low leachability form.
Radioactive waste solutions are obtained in most conventional separation processes in which uranium, plutonium, or other radionuclides are recovered from irradiated nuclear fuels. Recovery methods are usually based on solvent extraction, on precipitation, or on ion exchange techniques. The aqueous waste solutions left after the separation processes contain the bulk of the radioactive fission products in a highly dilute form, salts that have been added and possibly reducing or oxidizing agents that were added for the conversion of actinides from one valence to another.
Disposal of liquid radioactive waste to the environment is undesirable since the wastes continue to release dangerous radiation for thousands of years. Liquid radioactive wastes are sometimes highly acidic and corrode or destroy containers, even those made of stainless steel or other resistant materials, after a very long period of time. For this reason, it is undesirable to bury liquid waste in the ground due to the possible contamination of ground waters or to dispose such waste at sea.
It is necessary to reduce the bulk of the waste solutions and to convert the radioactive fission products into water insoluble form. Prior art has attempted to accomplish this in a number of ways, such as by dehydration and calcination, as taught in the U.S. Pat. No. 3,008,904; solidification of the radioactive waste as taught by U.S. Pat. No. 3,507,801; and the use of a fluidized bed to calcine the radioactive material as described in U.S. Pat. No. 3,862,296 and in the article "Technical and Economic Comparison of Methods for Solidifying and Storing High-Activity Liquid Waste Arising in the Reprocessing of Irradiated Fuel Elements from Water-Cooled and Water-Moderated Reactors" from the Symposium on the Management of Radioactive Wastes from Fuel Reprocessing of the Organisation for Economic Co-Operation and Development in Paris, dated March, 1973. The prior art processes, while effective in reducing the volume of waste material and the problem of the corrosive nature of the waste, still have certain inherent drawbacks, particularly as regards the use of the fluidized bed to calcine the waste material. The fluidized bed resulted in a product which was finely divided and susceptible to leaching when exposed to water. Production of a granular product without excessive fines requires that the introduction of feed be closely controlled to produce particles within a narrow size range and that the elutriation of fines be kept low.
It has been proposed to calcine the radioactive waste, mix it with glass frit, e.g., a borosilicate glass frit, and then melt the mixture to form a mass of glass in which the radioactive material is dispersed. This produces a product which is very resistive to leaching. Such as process is disclosed in U.S. Atomic Energy Commission (or Energy Research and Development Administration) Report BNWL 1667. However, to secure a uniform product, it is necessary to mix the calcine and frit. The highly radioactive character of the calcine makes it necessary to have specialized mixing equipment, which adds to the cost and complexity of the plant.